A motor controller for controlling the driving of a brushless motor is generally constructed so as to control the supply of a motor electric current according to an output of a rotation angle sensor which detects the rotation angle of a rotor. A resolver which outputs a sine-wave signal and a cosine-wave signal for the rotor rotation angle (electrical angle) is typically used as the rotation angle sensor. However, the resolver is expensive, and requires a greater number of wirings and a greater installation space. This disadvantageously prevents the cost reduction and the size reduction of an apparatus including the brushless motor.
To cope with this, a sensorless driving system is proposed, which is adapted to drive the brushless motor without using the rotation angle sensor. In the sensorless driving system, the phase of a magnetic pole (the electrical angle of the rotor) is estimated by estimating an inductive voltage occurring due to the rotation of the rotor. When the rotor is stopped or rotated at a very low speed, it is impossible to estimate the inductive voltage, so that the phase of the magnetic pole is estimated by other method. More specifically, a sensing signal is inputted to a stator, and a response of the motor to the sensing signal is detected. The rotational position of the rotor is estimated based on the motor response.